1. Field of the Invention
The invention generally relates to a rotary valve for an automatic transmission, comprising a spool rotatably received within a valve housing and adapted to be rotatively driven by an actuator, so that a hydraulic pressure from a hydraulic pressure source is selectively supplied to each of a plurality of frictionally engaging elements through the spool.
2. Description of the Prior Art
There are such conventionally known rotary valves for automatic transmissions as described, for example, in Japanese Patent Application Laid-open No. 312577/88.
Such a prior art conventional rotary valve for the automatic transmission suffers from a problem that line pressure supplied from the hydraulic pressure source through an input port formed in the valve housing is applied to an outer side surface of the spool from a radial direction and, for this reason, an opposite outer side surface of the spool is urged against an inner peripheral surface of a valve bore of the valve housing and, as a result, smooth rotation of the spool is prevented.
To avoid such a disadvantage, it can be conceived that a circumferential groove, to which the line pressure is transmitted, is formed in an outer peripheral surface to balance side force. However, the provision of such a circumferential groove results not only in an increased fabrication cost for the spool, but also in a problem of sealing between an oil passage opened into the peripheral surface of the spool and the Circumferential groove. Therefore, it is necessary to prolong the axial size of the spool to insure a sealing length, and to reduce the clearance between the spool and the valve housing to prevent leakage of oil. Such a construction, as described above, causes new problems that the entire length of the spool is increased, resulting in an increase in size of the entire rotary valve, and frictional force between the spool and the valve housing is increased. Hence, the size of a driving pulse motor must be increased.
In the prior art rotary valve, in order to cope with the situation where the spool becomes non-rotatable for any reason, such as, a malfunction of the pulse motor, for rotatively driving the spool, and a clogging with dust particles, measure is taken, such as, means for providing small vibration to the spool for preventing any sticking of spool, and a provision of an emergency valve. This causes a complication of the structure and an increase in cost. In addition, the spool of the rotary valve includes a plurality of axially arranged oil passages and, therefore, if the distance between adjacent oil passages is diminished, to reduce the axial size, it is necessary to conduct precise axial positioning of the spool.
Further, in the prior art rotary valve, when hydraulic pressure is being supplied to an intended frictionally engaging element, the hydraulic pressure for the other frictionally engaging elements is released through the rotary valve. The releasing of the hydraulic pressure is carried out through a common path during either downshifting or upshifting. In order to moderate shifting shock during downshifting and upshifting, it is desirable that during upshifting, the releasing of the hydraulic pressure from the frictionally engaging element, which has been in engagement theretofore, is retarded to permit the two engaging elements to be brought into a co-engagement. It is desirable that during downshifting, the releasing of the hydraulic pressure from the frictionally engaging element, which has been in engagement theretofore, is expedited to temporarily produce a neutral condition, thereby increasing the number of revolutions of engine to a proper extent. In the prior art rotary valve, however, the path through which the hydraulic pressure is released is common during both upshifting and downshifting and, therefore, it is difficult to provide different dropping characteristics of the hydraulic pressure during upshifting and downshifting.